Isao Souma

High-temperature creep and resultant anisotropy in ultrasonic velocity in isotropic graphite

Plastic deformation of isotropic graphite in the vicinity of 2500 °C was studied using an Instron-type testing machine. The load-deflection curve was found to show a proportional limit at high temperature. Creep curves of graphite were also measured and were simulated by an empirical equation based on parabolic creep. Following the creep tests, ultrasonic velocity in the elongated graphite was measured. The velocity showed 5%–20% decrease after the creep. Below 2500 °C, the rate of velocity decrease along the elongation axis was always larger than that across the elongation axis. Above 2500 °C, this behaviour was reversed with the rate of velocity decrease across the elongation axis being the larger. The rate of decrease appeared to depend firstly on temperature and secondly on stress.